For any given vehicle, transmission gear ratios are selected to satisfy performance requirements of gradability, fuel economy, acceleration, and ease of operation. Fuel economy considerations are essential to the selection of gear ratios. Numerically too high a gear ratio causes the engine to run too fast and thus, failing to operate at optimum fuel efficiency. Alternatively, an extremely low numerical gear ratio adversely effects vehicle performance, such as acceleration. Hence, acceleration is also an important concern in gear ratio selection. To maximize acceleration, gear ratios should be selected such that upshifts occur only when the next higher gear will provide more torque to the drive wheels.
Providing maximum torque to the drive wheels in each gear requires consideration of the ratio steps. Wide ratio steps in the lower gears provide for favorable fuel efficiency due to less frequent shifting. However, shifting through wide ratio steps requires skillful execution, otherwise a loss in fuel efficiency occurs. Alternatively, narrow ratio steps keep the engine running at or near full speed, thus supplying near maximum torque to the drive wheels. By skipping gears in narrow ratio step configurations, what is commonly referred to as skip shifting, a driver can simulate driving a wider step configuration, thus increasing fuel efficiency. Hence, selection of transmission gear ratios requires a balancing of various performance factors with the skill required of the end user.
Implementing ratio steps that decrease from low gear to high gear typically help to balance these performance factors. This decreasing trend between ratio steps illustrates what is called progressively smaller ratio steps. Progressively smaller ratio steps lend to maximum vehicle performance and maximum fuel efficiency, while providing a linear feel to the shifting process.
Historically, transmission designers have subjectively implemented progressive ratio steps. This subjective approach however, fails to produce truly uniform progressive ratio steps, meaning ratio steps which uniformly decrease from low gear to high gear. Thus, there stands a desire to eliminate the guesswork and subjectivity involved with selecting ratio steps. More specifically, there stands a desire to objectively select uniformly progressive ratio steps in order to maximize vehicle output efficiency.